Eosinophils are innate immune leukocytes associated with asthma, allergies and other diseases. Despite the long recognition of eosinophilic involvement in asthma, specific, non-redundant roles for eosinophils in disease pathogenesis had been elusive. However, recent studies from several laboratories have revealed new functions of eosinophils in immunomodulation and airway remodeling, changing the classic paradigm of disease pathogenesis and reinvigorating the field with a promise of more specific therapeutic targets. Our overall goal is to identify and mechanistically define effector functions of eosinophils vital to the initiation and exacerbation of asthma. In line with this goal, this proposal builds upon our novel discovery that mature human eosinophils express fully functional Notch ligands and receptors, indicating eosinophils utilize Notch signaling pathways, both as signal-receiving target cells and as signal- sending signaling cells. Our preliminary data establishes our overlying hypothesis that Notch signaling underlies eosinophil functions critical to asthma. Our studies will specifically investigate two hypotheses: 1) Notch receptor activation is required in parallel with cytokine signals to achieve full and sustained activation of eosinophils in asthmatic airways; and 2) Eosinophils promote a Th2 milieu in asthma by Notch ligand-mediated juxtacrine interactions with T cells. While experimental approaches utilize predominantly human eosinophils, proposed studies also take full advantage of the manipulative benefits of mouse models. Our proposal may provide vital insights into the mechanistic basis for eosinophil functions pertinent to asthma, and by extension other inflammatory diseases of the lung involving eosinophilia, relevant to development of novel, targeted therapeutic approaches.